In a common apparatus for biological sewage treatment (also called as “sewage bio-treatment”, “sewage biological treatment” or “biological treatment of sewage”), a mixed liquor is aerated in an aeration tank, then separated in a sedimentation tank to form a supernatant (also called as “water outflow”) and a concentrated mixed liquor (also called as “sludge” or “active sludge”); and, at least a part of the concentrated mixed liquor returns to the aeration tank (also called as “sludge return”) and is mixed with a sewage feed (also called as “water inflow”). Usually, sludge return is achieved by using a sludge return pump (e.g., a centrifugal pump). However, sludge return pump needs extra energy and frequent maintenance. In addition, an electric sludge return pump needs further insulating treatment to prevent leakage of electricity. Thus, a sludge return pump has to be disposed at a specific place, which restricts the flexibility of process lay-out, and usually needs a large amount of pipelines for connection, thereby increasing equipment investment and reducing reliability of the whole apparatus. These problems are especially serious in tower-type biological sewage treatment apparatus which has a small floor area and a great height.
Chinese patent application CN102372379A discloses a tower-type biological sewage treatment apparatus, comprising an upflow sludge bed reactor, an aerobic reactor disposed above the upflow sludge bed reactor, and a separator disposed in upper part of the aerobic reactor, in which both of aeration gas and aerated mixed liquor from the aerobic reactor enter a gas-liquor separating tank, and the separated mixed liquor enters a solid-liquor separating zone and is separated to form a supernatant and a concentrated mixed liquor, and the concentrated mixed liquor returns to the aerobic reactor and is mixed with a sewage feed and aerated to form the aerated mixed liquor.
However, when separating an aeration gas and a mixed liquor, or separating a mixed liquor to form a supernatant and a concentrated mixed liquor, a separating device or method in the art is usually unable to achieve desired separation effects. Especially, in a tower-type biological sewage treatment apparatus with limited volume and height, the separation to form a supernatant and a concentrated mixed liquor in a sedimentation tank is difficult, while poor separation even results in sludge floating and breaks sewage treatment process.
Further, during biological sewage treatment process, an air blower for aeration is usually a primary energy-consuming device, and an aeration gas is generally discharged directly to atmosphere after aeration treatment, so that the energy and oxygen in the aeration gas are not sufficiently utilized, which results in higher energy consumption in biological sewage treatment.
Hence, apparatus and method for biological sewage treatment with lower energy consumption, high reliability and high efficiency are still in need.